The invention relates to an optically readable disc comprising a round transparent substrate to be driven about a central axis of rotation, and an information layer.
Such discs are known in various types and versions. For example, British Patent Specification No. 1,446,009 (herewith incorporated by reference) describes optically readable video discs comprising a transparent substrate which is provided with a structure on one side which can be read through the substrate by means of a laser beam, and which is coated with a reflecting vacuum-deposited metal layer coated with a protective layer of lacquer. Frequently, two such discs are glued to each other back-to-back, yielding a double disc which is optically readable on both sides. For centering and positioning the video disc on a video-disc player, the disc is provided with a central aperture which cooperates with a centering member arranged on a drive spindle in the video-disc player. Optically readable audio discs generally have small dimensions and a differently coded information structure, but they are otherwise in principle identical to these known video discs.
Another type of optically readable disc is known from U.S. Pat. No. 4,074,282 (herewith incorporated by reference). These known discs comprise one or more transparent substrates--sometimes in combination with a non-transparent disc--which are glued to each other by means of coaxial annular spacers. By means of the annular spacers at least one gas-tight space is formed between the various parts of the disc. This type of optically readable disc is intended as a storage disc. By means of a laser beam a recording layer adjoining a sealed space can be provided with information through a transparent substrate. The recording layer comprises, for example, a thin layer of a tellurium alloy formed on the substrate by vacuum deposition.
Still other known optically readable discs are used as an intermediate product in the manufacture of optically readable video discs and audio discs. These otpically readable discs, generally referred to as "masters", comprise a glass substrate on which a photosensitive layer is deposited on one side. In a highly dust-free and very well ventilated room the video information and/or audio information is inscribed in the recording layer by means of a laser beam using special precision equipment. After the information has been recorded the photosensitive layer is developed. The photosensitive layer then contains a pattern of pits which can be duplicated by means of duplicating techniques of the type as described in for example U.S. Pat. No. 4,312,823 (herewith incorporated by reference). The masters are provided with a central aperture for accurate centering on the drive spindle of the machine used for recording the information.
The optical storage discs as generally used until now comprise two glass substrates which are glued to each other with interposed concentric annular spacers. However, when depositing the tellurium layer on these glass substrates and when depositing the photosensitive layer on the glass substrates of masters some problems occur. These problems are mainly associated with the method of depositing these layers on the substrates. By way of example a brief description will be given of the method used for depositing the photosensitive layer on the masters and the further processing of the exposed disc.
A ground glass substrate is employed whose surface, which is to be provided with the photosensitive layer, is first thoroughly cleaned and washed with demineralized water. These operations and all further operations to which the masters are subjected must be carried out in a conditioned highly dust-free room. For cleaning and washing the substrate is placed on a rotating spindle which projects through the central hole. The disc is dried by rotating it with a higher speed, so that the water particles are spun off the surface. The next steps in the method are also effected with a rotating disc, repeated drying being effected by rotation with a higher speed. After having been washed and dried the substrate is degreased by means of a suitable liquid, after which an adhesive liquid is applied to the disc. Then degreasing is repeated and finally the photoresist layer is applied. The photoresist layer is distributed evenly over the surface by fast rotation of the disc. After the exposure of the photo-resist layer on the described above, special machine developing takes place, again with the disc rotating, after which in a final operation the surface is rinsed again and subsequently a silver layer is applied by vacuum deposition. It has been found that it is difficult to prevent residual liquids used in previous steps of the method from settling in the central aperture of the disc. This may cause contamination of the product and consequent rejection of the product. Without describing the entire method of manufacturing optical storage discs it will be evident from the foregoing that, since the substrates of such discs also come several times in contact with liquids, similar problems will occur during manufacture.
Even with video discs and audio discs the central disc hole may give rise to problems. As already stated, the discs are provided with an extremely thin metal layer, in general vacuum-deposited silver, which is covered with a lacquer coating of 5 to 10 microns thickness. The silver layer must be coated very thoroughly because in the long run it may be affected by sulphur from the atmosphere, thereby rendering the disc unserviceable. The lacquer coating extends from the circumference of the disc to the central hole. At these locations there is an increased risk of the silver layer being attacked. Near the center hole this risk even increases because the disc is clamped in position near the central hole, so that at this location forces are exerted on the disc which may give rise to shear stresses between the lacquer coating and the silver layer.